1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for producing the same. In particular, the present invention relates to a liquid crystal display device using a liquid crystal partitioned by polymer walls as a display medium and a method for producing the same.
2. Description of the Related Art
Conventionally, various display media have been used for liquid crystal display devices. Examples of a display mode used for the display media include a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, a ferroelectric liquid crystal (FLC) mode, and a scattering mode such as a polymer dispersed type liquid crystal (PDLC) mode.
In recent years, in order to enhance the light utilization efficiency of devices, the PDLC mode has been receiving particularly attention among the above-mentioned display modes. As the PDLC mode, a system which electrically controls a transparent state and an opaque state, using the birefringence of a liquid crystal, is known. According to this system, basically, the ordinary refractive index of liquid crystal molecules in a display medium is made identical with the refractive index of a polymer which is the display medium, and the liquid crystal molecules are aligned in the direction of an electric field under the application of a voltage so as to display a transparent state. In contrast, the alignment of the liquid crystal molecules is disturbed under no application of a voltage so as to display a light scattering state.
However, the PDLC mode has a problem that a threshold voltage for driving a liquid crystal increases.
Japanese Laid-Open Patent Publication No. 5-257135 discloses a liquid crystal display device which locally contains a polymer with a network structure in a desired pattern as a display medium interposing a liquid crystal region and a region in which the alignment of liquid crystal molecules in the liquid crystal region is fixed by polymerization of a polymerizable liquid crystal between a pair of substrates. Such a polymer with a network structure can be formed by irradiation with light having a specific wavelength to the polymerizable liquid crystal material containing a reactive group such as a diacrylate group through a photomask.
In conventional liquid crystal display devices, an unnecessary voltage is applied to positions where electrodes or conductive tracks cross each other, whereby an undesired image is formed in these positions. In contrast, the polymer with a network structure in the above liquid crystal display device is formed in these positions by locally polymerizing a polymerizable liquid crystal material. Therefore, the alignment of a liquid crystal material in theses positions is fixed, and a switching threshold of the liquid crystal material can be locally enhanced. As a result, images are not likely to be formed in these positions under the application of a voltage.
However, the above-mentioned liquid crystal display device does not have sufficient shock resistance with respect to external pressure, while being capable of locally changing a switching threshold. Furthermore, the polymerizable liquid crystal material is generally highly reactive, and the polymerization thereof is considered to proceed with time in a region which is not irradiated with light. Therefore, the switching threshold, the alignment of liquid crystal molecules, and the like may gradually change in this liquid crystal display device.
Japanese Laid-Open Patent Publication No. 6-301015 discloses a liquid crystal display device 800 as shown in FIG. 8B. The liquid crystal display device 800 is produced in the following manner: As shown in FIG. 8A, a mixture 813 containing a liquid crystal material, a photopolymerizable resin, and a photopolymerization initiator is injected between a pair of substrates 801a and 801b. Then, a photomask 814 having light-blocking portions 810 and light-passing portions 811 is placed on the substrate 801a in such a manner that the light-blocking portions 810 cover pixel portions. Under this condition, the mixture 813 is irradiated with UV-light 808 through the photomask 814. As a result, as shown in FIG. 8B, liquid crystal regions 806 are formed in the pixel portions which are not irradiated with light, and a polymer wall 807 is formed in portions irradiated with light other than the pixel portions. Thus, the liquid crystal display device 800 having a liquid crystal medium layer is produced.
However, during the light irradiation in the course of producing the liquid crystal display device 800, the difference in optical path corresponding to the thickness of the substrate 801a is formed between the photomask 814 and the portions where polymer walls are formed. Thus, polymer walls which are wider than the light-passing portions of the photomask 814 are formed by light scattering, unless the light which is radiated to the substrate 801a is precisely parallel. Furthermore, even if parallel light is radiated to the substrate 801a, an unreacted photopolymerizable resin or the resultant polymer may remain in liquid crystal in the portions of the liquid crystal display device 800 which are light-blocked by the photomask 814, depending upon the type of photopolymerizable resin to be used. This decreases the display characteristics of the liquid crystal display device.
Japanese Laid-Open Patent Publication No. 7-287241 discloses a liquid crystal display device 900 as shown in FIG. 9B. The liquid crystal display device 900 is produced in the following manner: As shown in FIG. 9A, a transparent electrode 902a made of a material (e.g., ITO (Indidium Tin Oxide)), which is not likely to pass light with a specific wavelength, is formed on one surface of a substrate 901a, and a transparent electrode 902b made of the same material is formed on one surface of a substrate 901b. Then, the substrates 901a and 901b are placed in such a manner that the transparent electrodes 902a and 902b are opposed to each other. A mixture at least containing a liquid crystal material, a photopolymerizable resin, and a photopolymerization initiator is injected between the substrates 902a and 902b. The mixture is irradiated with light 908 having a specific wavelength through the substrates 901a and 901b. As a result, as shown in FIG. 9B, liquid crystal regions 906 are formed in pixel portions which are not irradiated with light, and a polymer wall 907 is formed in portions which are irradiated with light other than the pixel portions. Thus, the liquid crystal display device 900 having a liquid crystal medium layer is produced. Accordingly, in the liquid crystal display device 900 described in Japanese Laid-Open Patent Publication No. 7-287241, the transparent electrodes work as a photomask which reduces or blocks light having a specific wavelength.
However, in the production of the liquid crystal display device 900, the pixel portions composed of the liquid crystal regions 906 are also irradiated with light. Therefore, the resultant polymer remains in the pixel portions and may degrade the alignment of liquid crystal molecules and/or form insufficient pixel portions. This decreases the display characteristics of the liquid crystal display device.
When the intensity of the light radiated to the pixel portions is reduced for the purpose of solving the above-mentioned problems, the ratio of the intensity of transmitted light between the pixel portions and the portions other than the pixel portions decreases. As a result, unreacted photopolymerizable resin remain in liquid crystal after phase separation, causing the alignment defect of liquid crystal molecules.